Session A27: Vortex Dynamics: General I

Vortex Ring Interactions with Hemicylindrical Cavities

Tracheoesophageal (TE) speech is the technique of restoring voice to a patient following a laryngectomy. During TE speech the flow is characterized by periodic vortex rings impinging on the posterior wall of the trachea. That is, the fundamental interaction is characterized by a vortex ring impinging on a hemicylindrical cavity. This scenario is relevant to a number of fundamental fluid mechanics problems. The objective of this work is to experimentally identify, for the first time, the primary physics of a vortex ring impinging on hemicylindrical cavities of varying radius. A piston-cylinder vortex generator in a water tank was used to create a vortex ring with a formation number of F = 2.67 and Reynolds number of Re_Γ = 1450. Five different ratios of vortex ring radius (R_v ) to hemicylindrical cavity radius (R_cyl ) were examined, namely, γ = 1/4, 1/3, 2/5, 2/3.  Planer laser induced florescence (PLIF) and color dye flow visualization technique were employed to visualize the primary interactions. Flow visualization data qualitatively reveal how surface curvature of the hemicylindrical cavities affects the radial spreading of the primary vortex ring, and the subsequent inducement and separation of wall-bounded vorticity. Comparison with the physics arising due to vortex ring impingement on axisymmetric concave cavities (i.e., hemispheres) is also established in this study.

    

The topology of the flow induced by two and three satellite vortices in a bucket.

Many tornadoes include smaller, rapidly spinning subvortices or suction vortices. When such vortices are present, narrow traces of extreme destruction can be observed alongside the path of the tornado's center, where a weaker level of destruction is found. We will present a laboratory experiment mimicking such multiple-vortex tornadoes. The investigation involves a shallow layer of water driven into rotation by a rotating disc at the bottom of an open stationary cylindrical tank. This fluid layer exhibits a system of two and three satellite vortices equally distributed on a circle, located at the vertices of a polygon. The distribution of vorticity along the diameter of these satellite vortices is quasi-parabolic. We will report on the flow topology and the advection of passive particles using a dynamical systems approach. Poincare\'{e} maps and the finite-time Lyapunov exponent (FTLE) fields reveal the fluid exchanged between different flow regions surrounding elliptic and hyperbolic points. (edited) 

 

    

Experimental Study of Flow-Induced Oscillations of Flexible Structures with Non-Circular Cross Sections

Flow-induced oscillations of flexible structures with non-circular cross sections, including a square prism and symmetric hydrofoil with a NACA 0021 profile, are studied experimentally over a range of flow velocities. The flexible structures are made of silicone rubber and are strung horizontally across the test section with the flat side of each structure, i.e. the side length of the square prism and chord length of the hydrofoil, placed perpendicular to the direction of the incoming flow. Experiments are conducted in a recirculating water tunnel whereby displacement of the structure is recorded using high speed cameras in the crossflow and inline directions. Displacement along the length of each structure is then quantified using an in-house MATLAB code. Performed simultaneously with the displacement measurements, hydrogen bubble flow visualization reveal the vortex shedding patterns observed in the wake of the flexible structure. Various types of oscillations are observed depending on the structures’ cross section and flow velocity, including vortex-induced vibrations, galloping and mono- and multi-frequency responses at low- and high-mode-numbers.

    

Blockage effect on the wake of a rotationally oscillating linearly tapered cylinder

The wake of a linearly tapered cylinder executing rotational oscillations constrained by two stationary walls is studied experimentally at Reynolds number (based on mean diameter D_m) of 250. The tapered cylinder is forced to perform rotational oscillations at various oscillation amplitudes and normalized forcing frequencies. Tapered cylinders are characterized by taper ratio defined as the ratio of the length to the difference in diameters at the two ends and the cylinder used in the present study has a taper ratio of 70:1 and a mean diameter D_m of the cylinder is 8mm. Laser Induced Fluroscence technique is used for flow visualization of the wake structure. The normalized gap distance between the walls were T/D_m=2,4,6 for the present study. For T/D_m=2, it was found that the vortex shedding is supressed till FR<=0.4 and with further increase in FR, vortex shedding starts. At high forcing frequencies (FR>=2.5) vortex shedding again ceases. At T/D_m=4, the range of forcing frequencies for which the vortex shedding occured widens and at T/D_m=6, the wake behaves like an isolated and unbounded rotationally oscillating tapered cylinder. PIV experiments were performed to validate the flow visualization results.   

Understanding Flows with Vortex Geometry

Vortex filament methods have long provided a simplified, intuitive picture of fluid mechanics. For example, quantities like energy and helicity can be understood in terms of the geometry of vortex lines. In practice, however, these ideas are often difficult to extend to analysis of experimental data. I will discuss some of my past work which used 3D imaging to understand the evolution of helicity through vortex reconnections. I will also discuss more recent work which seeks to extend these experimental techniques to more complicated flows, with the aim of understanding vortex evolution in complex environments.   

Evolution of Vortex Lines in Pipe Flow

We are investigating the evolution and stability of vortex structures in transitional pipe flow. We have designed and built an experimental setup to generate concentrated streamwise vortex lines in pipe flow. By tracking dye seeded into the vortex cores and randomly distributed tracer particles, we can reconstruct the flow and the precise evolution of the vortex geometry. We can furthermore study the distribution of energy and other physical quantities as the vortex tubes evolve. The goal of this project is to shed light on how vortices behave in the presence of walls and background flows, providing a new lens of analysis for modeling the turbulent transition and other complex flows.   

A PIV Study of Flow Dynamics in a Cold-Flow Replica of a Swirl Stabilized Plasma Torch

Experimental flow field measurements are required for validation and uncertainty quantification in the development of a multi-physics simulation of an inductively coupled plasma torch. For this purpose, an optically-accessible torch replica has been built and PIV measurements are being made on the cold flow field, which is present prior to plasma ignition. The replica consists of a tube that is approximately 250 mm in length with a 56 mm inner diameter. The flow is injected tangentially at the bottom of the tube through four 1.44 mm² jets, each of which has a jet Reynolds number of 9000, which gives a swirl number of S = 4.3. The flow exits through a 30mm diameter nozzle into the room. Both two-dimensional three velocity component (2D-3C) measurements at low repetition rate and time-resolved 2D two-component (2D-2C) PIV velocity fields are presented. The 2D-3C measurements are made along the full length of the replica torch body and show a complex swirl dominated flow that exhibits vortex breakdown within the tube. The average axial velocity profile is axisymmetric and exhibits two closely spaced and interacting shear layers, separating regions of reversing flow directions. The complexity of the flow is further illustrated in the nozzle exit region, where the flow direction reverses around the centerline, and gas is sucked back into the tube. Time-resolved 2D-2C PIV is used to investigate the unsteady flow dynamics of this swirling and shearing flow further.

    

Evolution of suction parameters on a large-amplitude pitching hydrofoil

The leading-edge suction parameter (LESP) is broadly applied to unsteady aerodynamic models, such as for predicting shear layer separation and leading-edge vortex shedding. The LESP evaluated through partial circulation and its relation to the shear layer dynamics were previously defined and examined in experiments for airfoils undergoing flow perturbations and dynamic stall at moderate to high angles of attack (below 30 degrees). The current study applies this definition to a hydrofoil pitching at large amplitudes (up to 120 degrees) in both quiescent flow and uniform flow conditions. The LESP values calculated from the particle image velocimetry flow field results validate the partial circulation approach in large-amplitude conditions where the shear layer dynamics noticeably differ from those of lower-amplitude cases. The partial circulation and the associated suction parameter are extensively defined and measured at the trailing edge to account for the interaction between the trailing edge and the flow in such large-amplitude circumstances. The relations between the suction parameters and the hydrodynamic loads are studied in a parameter space of variable pitching frequencies and amplitudes.   

Experimentally simulating the formation of polygonal patterns by systems of satellite vortices

In this study, the interaction of point vortices arranged in different spatial configurations is investigated to gain fundamental insights into the role of satellite vortices on the formation of polygonal vortex patterns in a rotating fluid. Point vortices are synthetically created in experiments by using a 3 by 3 multi-position magnetic stirrer. The velocity fields at the free surface are recorded via Particle Image Velocimetry (PIV) measurement. Of interest is the evaluation of how each point vortex arrangement affects flow coherent structures, as extracted using Lagrangian Descriptors, and hidden flow structures, as extracted using proper orthogonal decomposition. This study will provide essential fundamental knowledge on the formation of polygonal vortex patterns and their modal transitions.